Article, especially valuable and security document, comprising a security feature

ABSTRACT

A value or security document ( 2 ) contains a security feature ( 4 ) in the form of a film laser with an emitter layer ( 12 ) and preferably a protective layer ( 14 ). A spatial, periodic modulation ( 8 ), preferably of the surface of a substrate, permits fine adjustment of a laser wavelength within a range of possible wavelengths. This permits a coding of the security feature by different portions of different wavelengths.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/EP02/12959, filed Nov. 19, 2002.

FIELD OF THE INVENTION

This invention relates in general to an object, but in particular avalue and security document, having a security feature. Without limitingthe generality the following statements are to relate to a value andsecurity document in the form of a bank note, it being clear to theexpert that the statements relating to the security feature also applyto other objects to be secured, for example high-quality industrialproducts, spare parts, ICs and the like, as well as the packing thereof.

DESCRIPTION OF THE BACKGROUND ART

For checking the authenticity of bank notes (and other value andsecurity documents as well as objects in general), security features areincorporated which are detectable, partly without special aids andpartly with special equipment, for distinguishing authentic bank notesfrom forged bank notes. Examples of such security features orauthenticity features are e.g. watermarks, security threads, holograms,Kinegrams, fluorescent fibers and the like. The present invention isintended to provide a further security feature, for objects in generaland in particular for value and security documents, which can be usedalone or in conjunction with other security features.

SUMMARY OF THE INVENTION

To this end, the present invention provides an object, in particular avalue and security document, having a security feature whereby thesecurity feature has the following features:

-   a) an emitter layer for stimulated optical emission;-   b) a spatial periodic modulation, in particular height modulation;    and-   c) optionally at least one boundary layer adjacent to the emitter    layer.

The present invention therefore provides an object having a securityfeature in the form of a “film laser.”

Specifically, a DFB film laser (DFB=distributed feedback) is to be usedas a security feature here. A special, albeit per se known, feature ofthis security feature consists in this laser element's periodicmodulation which can extend in one direction or also in two directions.

Film lasers and the like are already known in different variants and fordifferent purposes, but there has hitherto been no suggestion to usesuch film lasers as security features. An important feature is theabovementioned periodic modulation which can be obtained e.g. bymodulation of the emitter layer or the boundary layer. In a preferredembodiment the boundary layer forms a substrate to the surface of whichthe emitter layer is applied. In a particularly simple way themodulation is formed e.g. as a height modulation by embossing forexample a PET substrate. The period length of the predefined modulationpermits specific and fine adjustment of the frequency of the laseroscillation.

DE 198 05 993 A1 shows a film laser having a structure which the presentinvention essentially makes use of. However, no special applications forsuch a film laser are indicated in the stated print.

With such a film laser, the material of the emitter layer is excited byoptical pumping. When the laser threshold is reached and exceeded, laserradiation arises with a wavelength which depends on the modulationperiod of the modulation, whereby a suitable choice of the modulationperiod permits fine adjustment of the particular desired laserwavelength.

There are further publications on the topic of “laser film or plasticlaser,” e.g. in LaserOpto 31 3/1999 p. 21/22; Spektrum der Wissenschaft10/1999 p. 12 ff. Possible applications for plastic film lasersaccording to these publications are e.g. high-luminosity flat screensand artistic applications.

DE 198 36 813 A1 discloses a value and security document havingoptically excitable dye for authenticity testing, whereby a carriermaterial has dyes embedded therein which together with the carriermaterial form a laser active element. Although this measure permits asecurity feature to be formed, this security feature is lacking theinventive peculiarity of a periodic modulation and therefore thepossibility of adjusting the wavelength of one or more portions of thesecurity feature to a desired value and thereby permitting a codingwhich comprises the position and/or the wavelength of a security featureportion.

U.S. Pat. Nos. 5,448,582 and 5,625,456 disclose dye lasers and a fewapplications for such lasers. The laser effect is based here onscattering effects, so that no directional laser emission is possible.Possible applications are equipping objects with such laser activematerial and detecting the presence of such material by irradiation witha suitable excitation wavelength.

As mentioned above, a coding can be obtained in the inventive securityfeature by accordingly adjusting the modulation. The modulation can beeffected e.g. by periodic modulation of the refractive index. This isrealized for example by a height variation of the emitter layer or aboundary layer. Furthermore, the spatial modulation can also be causedby a periodic variation of the thickness of the emitter layer. Becauseit is particularly simple to produce, however, a height modulation ofthe surface of the substrate adjoined by the emitter layer is preferred,whereby an aluminum layer can also be provided as a reflecting layer ormirror plating between substrate and emitter layer to amplify the lasereffect.

Two-dimensional application of the periodic modulation permits thedesired laser wavelength to be laid out redundantly in the total area ofthe security feature, but it is also possible to provide the modulationperiod or several modulation periods in a sequence differing in twodirections. Two-dimensional modulation permits the emissioncharacteristics to be adjusted more specifically. Unlike known value andsecurity documents with a laser effect as a security feature, only theinventive use of the modulation permits a coding that makes the securityfeature particularly valuable.

As per se known, the emission layer of the security feature can havedisposed thereon a protective layer with a lower refractive indexcompared with the emission layer. This guarantees total reflection inthe emitter layer even in case of impurities on the surface of thesecurity feature.

For improving the life time of the security feature the inventionprovides that at least one layer of the security feature is equippedwith optical absorbers and/or antioxidants and/or radical traps. Opticalabsorbers increase the layer stability of the laser active emitterlayer. The stated additional substances in one or more layers alsoincrease the stability of the fluorescent properties.

As mentioned above, the periodic modulation allows a simultaneouslysimple height modulation, obtained e.g. by embossing, and versatilecoding of the security feature. The coding can be formed according tothe invention in particular by

-   a) defined sites being assigned the state “laser active” or “laser    inactive”;-   b) defined sites being assigned the state “laser inactive” or “laser    active with one of a predetermined number of wavelengths”; and/or-   c) defined sites each being assigned their own wavelength from a    number of possible wavelengths of the laser light or the state    “laser inactive.”

The simplest type of coding in accordance with the above feature a)permits e.g. the coding of states such as “0” and “1,” i.e. a bit-by-bitcoding of the security feature. The second type of coding in accordancewith feature b) permits an extremely diverse coding. For example, in amatrix-shaped field the intersection points of the matrix can beequipped with laser active elements of different wavelengths and alsowith laser inactive areas. With three possible laser active states andone laser inactive state and for example 16 matrix intersection points,4**16 (=4¹⁶) different codings are then possible.

With the third coding possibility c), a field of laser active and laserinactive sites can be so coded that each site is assigned a certainlaser wavelength whereby the individual sites are either active orinactive.

The sites of the security feature for the coding can be disposed along aline; they can also be distributed two-dimensionally. For avoidingmutual interference, the sites, i.e. active areas, can have disposedtherebetween separating elements e.g. in the form of bars which arelaser inactive.

As mentioned above, the inventive security element can be realized verysimply. For example it can be formed as the substructure of anothersecurity feature, e.g. an embossed hologram. The security feature isthen created in the course of the usual production, whereby theformation of the hologram comprises the formation of the periodicmodulation.

In a special embodiment of the invention it is provided that thesecurity feature is incorporated into the object in the form of one ormore prefabricated portions, in particular in the form of planchets.Such planchets have for example diameters of approx. 0.5 to 5 mm, andare preferably incorporated into paper or applied to the surfacethereof. Alternative possibilities for incorporation or application ofthe security feature into or onto an object involve incorporating orprinting small portions of films as pigments in screen printing inks.Film lasers can also be independently produced and glued to the objectsto be equipped.

Objects equipped with the inventive security feature can be checked forauthenticity by bringing the object with the security feature into theproximity of a pumping light source. This can be in particular a pulsedlaser source which supplies enough energy to the security feature forlaser operation to start.

However, it is also possible to excite the security feature by electriccurrent. To this end the emitter layer is equipped with electricallyconductive layers to which a voltage is applied. In particular, suchelectrode layers can be the mirror plating formed by an aluminum coatingon the substrate side of the emitter layer and a transparent ITO layeron the other side of the emitter layer (ITO: indium tin oxide).

BRIEF DESCRIPTION OF THE DRAWINGS

In the following some embodiments of the invention will be explained inmore detail with reference to the drawing, in which:

FIG. 1 shows a cross-sectional view through a bank note having asecurity feature incorporated therein in accordance with the invention;

FIGS. 2, 3, 4 and 5 each show an alternative possibility for theformation of the layers shown in FIG. 1 within the detail “X”;

FIG. 6 shows a spectrum of an inventive security feature with peakspresent at different wavelengths within an amplification curve of theinventive security feature;

FIG. 7 shows a schematic plan view of a bank note having a matrix-shapedsecurity feature which is coded two-dimensionally;

FIG. 8 a shows a further embodiment of a security feature in the form ofa matrix with fields which can each emit laser light with a wavelengthassigned to the field within the matrix; and

FIG. 8 b shows a spectrum belonging to the security feature according toFIG. 8 a.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, a security feature 4 is incorporated into a valueand security document present as a bank note 2 here. The fundamentalstructure of said security feature 4 in the form of a DFB film laser(DFB=distributed feedback) is known, reference being made for example tothe abovementioned DE 198 05 993 A1.

The security feature 4 contains a PET substrate 6 (PET: polyethyleneterephthalate) having formed on the upper side thereof a heightmodulation 8 with a period length 1. The height modulation is formed byembossing the PET substrate, as described in detail e.g. in theabovementioned DE 198 05 993 A1.

In the present embodiment the height modulation is formed in only onedirection, i.e. from left to right in FIG. 1. It is assumed here thatthe hills and valleys of the structure of approximately sinusoidal crosssection extend with the same cross section into and out of the plane ofprojection. However, an alternative embodiment is also possible with aperiodic height structure in a second direction as well, in particularin a direction perpendicular to the first direction, i.e. a heightmodulation perpendicular to the plane of projection.

On the surface of the substrate 6 equipped with the height modulation 8there is an aluminum layer 10 which is e.g. vaporized on the substrate 6as the reflecting layer or mirror plating.

Above the aluminum layer 10 there is an emitter layer 12. Said emitterlayer is so formed that stimulated optical emission of laser light ispossible upon corresponding excitation by optical or electric energy.Materials that can be used for the emitter layer are e.g. fluorescentpolymers, a polymer layer with embedded laser dyes or inorganiclaserable pigments. The upper side of the emitter layer 12 likewise hasa height modulation whose modulation period is likewise l, identicalwith the modulation period 1 of the substrate surface. The heightmodulation 16 of the emitter layer 12 is adjoined upwardly by aprotective layer 14 consisting of a material whose refractive index islower than that of the emitter layer 12. This guarantees totalreflection within the emitter layer even in the case of impurities ofthe film.

The emitter layer 12 and the protective layer 14 are provided withseveral materials that improve the properties for the purpose of asecurity feature. FIGS. 2 to 4 show the possible alternatives for suchsubstances situated in the layers. FIG. 5 indicates the meaning of thesymbols used in FIGS. 1 to 4. For example, according to FIG. 2 theemitter layer 12 contains radical traps (e.g. HALS systems; HALS:hindered amine light stabilizer), as well as antioxidants. Theprotective layer 14 contains not only the radical traps and antioxidantsbut also optical absorbers.

FIGS. 3 and 4 show alternative materials for incorporation into thelayers 12 and 14.

A peculiarity of the inventive security feature is the periodicmodulation of the laser active emitter layer 12. The modulation period 1of the height modulation 8 has an influence on the wavelength of thelaser light emitted by the emitter layer 12 during oscillation buildupof the laser. According to FIG. 1, excitation is effected by irradiationwith a pump light source, for example with a pulsed laser. The result ofoptical pumping is oscillation buildup of the laser active emitter layer12. The laser light exits for the most part perpendicular to the surfaceand to the modulation direction of the security feature.

The height modulation 8 can now be varied by varying the modulationperiod 1 continuously and/or in discrete steps or omitting it in certainsections. In the areas without height modulation there is no laseremission. In the other areas, termed “laser active” here, there isemission of laser light with a wavelength which can be selected withinlimits by adjusting the modulation period 1.

Upon excitation of the film (consisting in FIG. 1 of the substrate 6 andthe superjacent layers) with laser light there is, below the laserthreshold, merely a spontaneous emission of the laser dye contained inthe emitter layer. At energy above the laser threshold an emission lineforms which typically has a width of less than 1 nm. According to FIG. 1the laser beam exits perpendicular to the surface of the film.

According to FIG. 6 the amplification area shown by the “envelope”contains several possible laser wavelengths which can be adjusted by acorresponding choice of the modulation period 1. The differentwavelengths of the laser light permit different film lasers or filmlaser portions to be distinguished from each other. As mentioned, areaswithout any modulation are also possible, which are then “laserinactive.” A possible number for different discrete wavelengths withinthe amplification area of the laser is N =20.

FIG. 7 shows a possible form of a two-dimensionally coded laser film 104on a bank note 102. The laser film 104 is formed by a square filmelement with the edge length X2, comprising 16 square partial elementswith the edge length X1. The different 16 elements or portions of thelaser film 104 are each assigned one of three different wavelengths λ1,λ2 and λ3. This results in 3**16 (=3¹⁶) different coding possibilitiesfor the security element in the form of the laser film as shown in FIG.7. Adding the possibility “laser inactive” for each of the 16 portions,the result is 4**16 (=4¹⁶) coding possibilities.

The laser film 104 shown in FIG. 7 is scanned e.g. with an excitationlaser to “read” the wavelengths or the state of the individual portions.To this end a thin laser beam is guided in rows across the four columnsof the matrix array. Alternatively, row or column scanning of fourportions at a time can be effected with a linear laser beam, wherebysignal evaluation is done with the help of a multichannel spectrometer.

It can be seen that instead of the multidimensional matrix form one canalso choose a linear form with the states “laser active” for one ofseveral possible laser wavelengths or “laser inactive.”

FIG. 8 a shows an embodiment of a film laser 204 with four fields 220each being assigned a certain wavelength λ1, λ2, λ3 and λ4. In theembodiment according to FIG. 8 a the fields for the wavelengths λ1, λ3and λ4 are “laser active,” the field for the laser wavelength λ2 “laserinactive.” Evaluation of the film laser excited to laser emission byexcitation light results in an intensity spectrum according to FIG. 8 b.

Not shown in FIGS. 7 and 8 a is the special feature of providingseparating areas between the individual portions of the matrix array toprevent mutual influence of the laser wavelengths upon reading of thecoding.

In a further embodiment, electric energy can be used for exciting theemitter layer 12 instead of pumping light in accordance with FIG. 1. Forexample, the aluminum layer 10 which functions as a mirror plating canbe formed at the same time as an electrode to which one pole of avoltage source is connected. In the area of the height modulation 16between the layers 12 and 14 a transparent ITO layer can be providedwhich is connected to the other pole of the voltage source if laseroscillation is to be built up. It is also possible to use two metalelectrodes with a hole or grid structure which are permeable to theexiting laser light.

Alternatives to the above-described embodiments have already beenmentioned. It is thus possible e.g. to provide a modulation of therefractive index of the emitter layer instead of the height modulationof the substrate. The modulation is important for obtaining a certainlaser wavelength or several certain laser wavelengths to permit acoding.

1. A value and security document, comprising a security feature having asubstrate, an emitter layer for stimulated optical emission and aperiodic height modulation formed on a surface of the substrate.
 2. Avalue and security document according to claim 1, wherein the periodicmodulation extends in a single direction.
 3. A value and securitydocument according to claim 1, wherein the periodic modulation extendsin two different directions.
 4. A value and security document accordingto claim 3 wherein said two different directions extend at right anglesto each other.
 5. A value and security document according to claim 1,wherein a reflecting layer is located between the substrate and theemitter layer.
 6. A value and security document according to claim 1,wherein the emitter layer has disposed thereon a protective layer with alower refractive index compared with the emitter layer.
 7. A value andsecurity document according to claim 1, wherein the emitter layer isequipped with at least one of optical absorbers, antioxidants, andradical traps.
 8. A value and security document according to claim 1,wherein the security feature comprises a coding, whereby the coding isformed by at least one of: a) defined sites being assigned a state“laser active” or “laser inactive;” b) defined sites being assigned astate “laser inactive” or “laser active with one of a predeterminednumber of wavelengths;” and c) defined sites each being assigned theirown wavelength from a number of possible wavelengths of the laser lightor a state “laser inactive.”
 9. A value and security document accordingto claim 8, wherein the sites are disposed along a line.
 10. A value andsecurity document according to claim 9, wherein the sites are disposedin two dimensional distribution.
 11. A value and security documentaccording to claim 8, wherein separating elements are formed between thesites.
 12. A value and security document according to claim 1, whereinthe security feature is formed as the substructure of another securityfeature.
 13. A value and security document according to claim 12 whereinsaid another security feature is a hologram.
 14. A value and securitydocument according to claim 1, wherein the security feature isincorporated into the object in the form of one or more prefabricatedportions.
 15. A value and security document according to claim 14wherein said prefabricated portions are planchets
 16. A value andsecurity document according to claim 1, wherein the security feature isprovided on the object by means of screen printing or glued to theobject as a prepared element.
 17. A value and security documentaccording to claim 1, wherein the emitter layer has a protective layerdisposed thereon that is equipped with at least one of opticalabsorbers, antioxidants and radical traps.